The present invention relates, in general, to electrochemical fuel cells and, more specifically, to combustors for heating a fuel processor.
Fuel cells have been used as a power source in many applications. Fuel cells have also been proposed for use as a vehicular power plant to replace the internal combustion engine. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode side of the fuel cell and air or oxygen is supplied as the oxidant to the cathode side. PEM fuel cells include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distribution the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the fuel for the vehicle owing to the ease of on-board storage of liquid fuels and the existence of a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a fuel processor, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam and methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide according to this reaction:
CH3OH+H2Oxe2x86x92CO2+3H2.
The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur. The heat required to produce enough hydrogen varies with the demand put on the fuel cell system at any given point in time. Accordingly, the heating means for the fuel processor must be capable of operating over a wide range of heat outputs. Heating the fuel processor with heat generated externally from either a flame combustor or a catalytic combustor is known. U.S. patent applications Ser. No. 08/975,422 now U.S. Pat. No. 6,232,005 and Ser. No. 08/980,087 now U.S. Pat. No. 6,077,620 filed in the name of William Pettit in November, 1997, and assigned to the assignee of the present invention, disclose an improved catalytic combustor, and the integration thereof with a fuel cell system which fuels the combustor with unreformed liquid fuel, hydrogen-containing anode exhaust gas from the fuel cell, or both. The operating cycle depends on many factors, such as anode stoichiometry, steam/carbon ratio, electrical demand placed on the system, etc.
Thus, it would be desirable to provide a method for controlling a combustor in a fuel cell system which makes efficient use of all available fuel. It would also be desirable to provide a method for controlling a combustor having dual fuel and multiple fuel composition inlet streams. It would also be desirable to provide a method for controlling a combustor having dual fuel and multiple fuel composition inlet streams and dual oxidant (air) inlet streams with differing oxygen content. It would also be desirable to provide a method for controlling a combustor having dual fuel and multiple fuel composition inlet streams which meets current vehicle emission requirements at all times during the fuel cell operation cycle.
A control method for a methanol tailgas combustor used in a fuel cell system in which some unused hydrogen from the anode (anode effluent), and unused oxygen from the cathode (cathode effluent) of a fuel cell stack are supplied as separate fuel and air streams to the combustor with selective quantities of fuel processor reformate and separate fuel and air supplies. The terms effluent and exhaust are used herein interchangeably.
In one aspect of the present invention, the control method comprises the steps of:
providing first and second fuel streams to the combustor, the first fuel stream being a hydrocarbon fuel stream, the second fuel stream consisting of reformate from the fuel processor and/or the anode effluent from the fuel cell;
providing first and second air flow streams to the combustor, the first air flow stream being from first air source, the second air flow stream being the cathode effluent from the fuel cell;
determining the power input requirement of the fuel processor;
determining the output power of the combustor to support the determined power input requirement of the fuel processor; and
regulating the quantity of at least one of each of the first and second fuel streams and at least one of each of the first and second air flow streams to the combustor to provide a power output from the combustor to meet the determined power output requirement of the fuel processor.
In one aspect of the present method, the regulating step comprises the utilization of all available second fuel stream and second air flow stream in the combustor prior to supplying any of the first fuel stream and/or the first air flow stream to the combustor.
In one aspect of the present method for the initial start-up of the combustor, the method comprises the step of before supplying the first fuel stream to the combustor, preheating a catalyst bed in the combustor to a predetermined operating temperature. In the start-up mode, the first fuel stream and the first air flow stream are exclusively supplied to the combustor unless an optional buffer or supply of reformate or hydrogen is available.
In a fuel processor start-up mode of operation, the present method includes the steps of:
before supplying a hydrocarbon fuel to the fuel processor, calculating the power requirements of the combustor to raise the temperature of the fuel processor to a predetermined warm-up temperature;
determining the heat content of the first fuel stream and the first air stream to the combustor to provide the determined power requirements of the combustor to raise the fuel processor to the predetermined warm-up temperature;
measuring the temperature of the fuel processor; and
regulating the quantity of the first air stream supply to the combustor to balance the enthalpy of the first fuel stream and the first air stream supplied to the combustor to supply heat energy at the desired temperature.
Preferably, the regulating step includes the step of regulating the quantity of the first air stream supply to the combustor by controlling the output flow of one or more valves in the first air stream supply or by varying the speed of the air compression device supplying the first air flow stream. The output flow of a valve is preferably adjusted by controlling the diameter of an output flow orifice of the valve. Alternatively, the output flow of a valve is adjusted by changing the position of the valve in the valve body from open to closed or to an intermediate position such as partially open or partially closed. It is most preferred to control the diameter of the output flow orifice of the valve.
After the fuel processor has reached a predetermined warm-up temperature sufficient to produce reformate from fuel and water, the present method comprises the steps of:
diverting all of the reformate from the fuel processor to the combustor;
determining the enthalpy of the reformate generated by the fuel processor;
determining the enthalpy of the combustor output attributed to combustion of the reformate in the combustor;
calculating the power requirements of the combustor to raise the temperature of the fuel processor to a predetermined start-up temperature;
calculating the difference between the enthalpy of the reformate diverted to the combustor and the predetermined power output requirements of the combustor;
determining the first air flow stream and the first fuel stream quantities to the combustor based on the calculated difference;
supplying the first fuel stream to the combustor in the determined quantity; and
regulating the first air stream quantity to the combustor to zero the difference.
In this latter aspect of the present method, the first fuel flow is regulated to the combustor based on the total output power requirement of the combustor and the enthalpy of the reformate supplied to the combustor from the fuel processor.
In another aspect of the present method, the method includes the steps of after the fuel processor temperature has reached the predetermined start-up temperature, diverting all of the fuel processor reformate to the fuel cell;
calculating the power requirement and operating temperature of the fuel processor during a run mode of operation;
communicating the output gas stream of the fuel processor to the fuel cell;
determining the enthalpy of the anode effluent and the cathode effluent from the fuel cell;
communicating the anode effluent and the cathode effluent to the combustor;
calculating the power requirements of the combustor to operate the fuel processor at the predetermined run temperature and power output;
calculating the difference between the enthalpy of the anode effluent and cathode effluent supplied to the combustor and the calculated power output requirement of the combustor;
calculating the first fuel stream and the first air flow stream requirements for the combustor based on the determined difference;
supplying the first fuel stream and the first air flow stream in the calculated quantities to the combustor; and
adjusting the quantity of the first air flow stream to the combustor to run the combustor at the preset operating temperature. However, the first air flow stream may not be required under normal operating conditions depending on cathode stoichiometry.
In this latter aspect of the invention, the control method adjusts both of the quantity of the first fuel stream and the first air flow streams applied to the combustor based on the determined difference.
In order to shut down the combustor operation, the control method determines if the supply of the first fuel stream to the fuel processor is discontinued and, if the first fuel stream is discontinued, sets the flow rate of the first air flow stream to the combustor to a preset flow rate to lower the combustor temperature to a preset shutdown temperature.
Alternately, if the first fuel stream flow is not discontinued, the method calculates the enthalpy of the anode effluent and the cathode effluent from the fuel cell, and adjusts the flow rate of the first air flow stream to the combustor to consume all of the anode effluent.
Finally, the present method, in another aspect in which the method executes a diagnostic loop, the method includes the steps of:
comparing at least one of a combustor catalyst bed temperature, a combustor vaporizer temperature, an inlet air stream temperature to the combustor, the anode effluent inlet temperature, a predetermined temperature rate of change of any of said temperatures, a vaporizer air flow rate, the pressure of the combustor, and the combustor air flow rate with respective limits;
determining if any of the measured parameters exceeds the respective preset limit; and
discontinuing operation of the combustor if any of the preset limits are exceeded.
In one aspect of the present control method where the fuel cell is deactivated, all of the fuel processor output stream is diverted to the combustor. Fuel from the first fuel stream, if necessary, and air from the first air flow stream, if necessary, are supplied to the combustor in quantities determined by the enthalpy determination to raise the temperature of the fuel processor to a desired operating temperature.
The present control method, in another aspect during continuous run operation of the fuel cell, by using enthalpy balance, completely utilizes all excess hydrogen and oxygen in the anode and cathode effluents from the fuel cell before any additional amounts of external fuel or compressor air are supplied to the combustor.
In another aspect, the present control method contemplates the regulation of the amount of air supplied to the combustor from an external source or an air compressor based on the determined power requirements of the combustor as demanded by the fuel processor and the amount of oxygen in the cathode effluent supplied to the combustor. The amount of the first air stream supplied from the compressor to the combustor is regulated to balance the enthalpy of the reactions in the combustor to support a given combustor heat output to the fuel processor.
The methanol tailgas combustor control method of the present invention regulates air flow from an external source to the combustor for enthalpy balance to provide temperature control within the combustor and a desired power or heat output at any operating level of the fuel processor. The present control method provides control of a combustor in a fuel cell apparatus to meet current and future governmental emission levels.